Process and plant for the hydrothermal treatment of asbestos and/or asbestos-containing materials in supercritical water

ABSTRACT

Herein described is a process for the hydrothermal treatment of asbestos and/or materials containing asbestos in supercritical water (Supercritical Water, SCW), that provides for the withdrawal ( 3 ) of water from a tank ( 2 ), the transformation ( 5 ) of the water in supercritical water, the reaction of the supercritical water with asbestos or with material containing asbestos in a suitable environment ( 8 ) by means of a hydrolysis process, the cooling ( 14 ) of the waste water, the filtering ( 15 ) of the waste water, the collection of the waste water in a tank ( 17 ). Herein is also described a plant for the treatment of asbestos and/or materials containing asbestos comprising a water tank ( 2 ), a withdrawal pump ( 3 ) associated to said tank ( 2 ), a furnace ( 4 ) containing a serpentine coil ( 5 ) fed by said withdrawal pump ( 3 ) for the transformation of the water in supercritical water and a reactor ( 8 ) for the reaction of the supercritical water with asbestos and/or with the material containing asbestos, heat exchange means ( 14 ) for cooling the waste water from said furnace ( 4 ), and filtering means ( 15 ) of the water positioned at the output of said exchange means ( 14 ) and collection means ( 17 ) of the cooled and filtered waste water.

The present invention refers to a process for the hydrothermal treatmentof asbestos and/or asbestos-containing materials in supercritical waterand relative production plant.

Numerous hydrate silicates, of various chemical compositions, withmicrocrystalline structure and a fibrous appearance are grouped underthe name of amianthus or asbestos. They are subdivided into two classes:

ANPHIBOLES (hydrate silicates of calcium, iron, sodium and magnesium) ofwhich the following are part CROCIDOLITE (or blue asbestos)Na₂(Mg,Fe)₇Si₈O₂₂(OH)₂ AMOSITE (or brown asbestos) (Mg,Fe)₇Si₈O₂₂(OH)₂Commercial name of the mineral GRUNERITE and CUMMINGTONITE ANTOFILLITE(Mg,Fe)₇Si₈O₂₂(OH)₂ ACTINOLITE Ca₂(Mg,Fe)₇Si₈O₂₂(OH)₂ TREMOLITECa₂Mg₅Si₈O₂₂(OH)₂

SERPENTINE (hydrate silicates of magnesium) of which the following arepart CHRYSOTILE (or white asbestos) Mg₃Si₂O₅(OH)₄ or 3MgO.2SiO₂.2H₂O

Asbestos has been used for years in a wide range of industrialapplications because of its special characteristics such as low cost,flexibility, sound absorption, and resistance to fire, heat and chemicaletching. Among the applications of greater significance we can mention:building materials, (the known “Eternit” contains 15% asbestos in thechrysotile form), the materials used as insulation in railway carriagesand in ships, covering for water ducts and air conditioning ducts,anti-friction materials in the linings of brakes in automobiles andrailway cars, the yarns for fabrics made for protective garments againstfire.

With the Italian regulations the waste materials containing asbestos areclassified as toxic-poisonous and the law that prohibits its extractionand import dates back to 1992. At national level their amount isestimated at not less than 15 million tons as asbestos has beengenerally used together with another material.

Currently there is particular interest in the operations and proceduresfinalised to recuperation and/or elimination of the asbestos and of theproducts that contain it. The recuperation interventions, that providefor the removal as well as the discarding of any product containingasbestos that has lost its use designation, that has been abandoned oris scheduled to be abandoned, produce Asbestos-Containing Waste (ACW),that contains a variable quantity of asbestos in the interval (10-100)%on weighted base.

A problem that is common to the recuperation and/or neutralisingprocesses of the ACW materials is that associated to the efficiency oftransforming the asbestos in inert products that are not dangerous, thatis in materials that no longer can be assimilated to fibres that can bebreathed in. In fact, for the purpose of evaluating the fibrousasbestos, whose danger is also linked to the dispersion in air, anyelongated thready or needle-like solid object is intended with a lengthequal to or greater than 5 μm, diameter less than 3 μm and with alength/diameter ratio equal to or greater than 3.

The diameter of a fibre takes on basic importance for the capability ofit being breathed in, while the length is not very significant as it isvery difficult to breath in the fibres which are longer than 200 μmwhile the short fibres are eliminated by the clearance of themacrophages.

A recent provision of Italian law (13/03/03) decrees that:

the waste of asbestos or of materials containing asbestos have to becollected in a dump for dangerous waste, specifically or fitted with aspecific cell, where thus it undergoes a process of encapsulation in thesite, (that is in the cell of the waste dump).

if the ACW materials have been submitted to treatment processes, suchas: stabilization, encapsulation, embedding, orchemical-physical-thermal treatment, the decree provides for them to becollected in a dump for non-dangerous waste.

Among the main processes known for the treatment of the ACW materials,only the chemical-physical technologies (Yoshiro et al., U.S. Pat. No.3,941,184; Block, U.S. Pat. No. 5,753,031; U.S. Pat. No. 5,753,032; U.S.Pat. No. 5,753,033; U.S. Pat. No. 5,753,034; U.S. Pat. No. 5,753,035;and Block et al., U.S. Pat. No. 5,743,841) and thermal technologies(Aspireco, European patent n. 0344563; Italian patent n. 20799-Mi/88)tend to eliminate the potential risk by transforming asbestos in anon-fibrous and non-toxic crystalline phase.

Because of the high costs, the dump is the most common removal process,even though it is less effective as it does not eliminate the asbestosproblem, seeing its potential danger remains unaltered.

Over recent years, methods for transforming thready asbestos have beenproposed (chrysotile and amosite) in inert materials (U.S. Pat. No.5,743,841) by means of neutralising the asbestos by etching at a lowtemperature and pressure. The procedure is based on a etching by acidsusing a mixture of a strong acid and a kind capable of generatingfluorine in the system. The acid has the task of demolishing thestructure of the asbestos hydrolysing the MgO groups, while the fluorineshould etch the “silicate” component of the asbestos structure.

The interest for these treatments lies in the fact that they can beeasily applied on site directly on the manufactured articles containingasbestos (for example, tubes covered with asbestos-based insulation,whose percentage is around 12%), and therefore there are no problems intransporting dangerous material.

The disadvantages of these treatment methods can be summed up asfollows:

the use of a dangerous reagent, hydrofluoric acid, that requiressuitable measures of prevention and safety to be adopted;

the transformation takes a long time. In fact, the kinetics of theneutralising process by etching depends on two critical factors:

1. the velocity of penetration (imbibition) of the watery solutioninside the manufactured article containing asbestos;

2. the wettability characteristics of the manufactured articlecontaining asbestos.

Because of these two critical factors the time needed for neutralisingis around days. In fact, as given in Block's patent (U.S. Pat. No.5,743,841) starting from a acid/chrysotile ratio (cementitious mix with11.7% content of chrysotile) of 1.5:1, the process is basicallycompleted after two days of treatment; in particular Block shows thatfrom the XRD analyses (X-Ray Diffraction) it results that the residualchrysotile after a day is 0.5%, being reducing to 0.1% after 4 days; thetreatment is, substantially, a superficial treatment, because of theproblems of penetration of the watery solution inside the porousmaterial to be treated.

The object of the present invention is to provide a treatment procedureand a relative plant that are economically advantageous and that allowthe neutralising of asbestos and/or materials containing asbestos andthe elimination of the problems associated with the penetration process(imbibition) using a supercritical and oxidising environment(supercritical water, SCW).

In accordance with the present invention this object is achieved bymeans of a process for the hydrothermal treatment of asbestos and/ormaterials containing asbestos in supercritical water (SupercriticalWater, SCW) characterised in that it provides for the following steps:

withdrawal of water from a tank;

transformation of the water into supercritical water;

reaction of the supercritical water with asbestos and/or with thematerial containing asbestos in a suitable environment by means of ahydrolysis process;

cooling of the waste water;

filtering of the waste water;

collection of the waste water in a tank.

In accordance with the present invention this object is also achieved bymeans of a plant for the treatment of asbestos and/or materialscontaining asbestos characterised in that it comprises a water tank, awithdrawal pump associated with said tank, a furnace containing aserpentine coil fed by said withdrawal pump for the transformation ofthe water into supercritical water and a reactor for the reaction of thesupercritical water with asbestos and/or with the material containingasbestos, heat exchange means for cooling the waste water of saidreactor, water filtering means placed at the output of said exchangemeans and collection means for the cooled and filtered waste water.

The operative conditions are preferably the following:

400° C.<T<750° C.;

22.11 MPa<P<28 MPa.;

hydrolysis time <24 hours.

The environmental, energy and productive advantages that would beobtained with the new hydrothermal treatment of asbestos or of materialscontaining asbestos in supercritical water are multiple andunquestionable.

The hydrothermal process of hydrolysis can represent an economical andfinal solution for elimination as, in comparison to the known thermalprocesses, it presents greater potential in reducing the costs oftreatment for the “low” working temperatures and the possibility ofcarrying out effective energy recuperation in the process. Theadvantages and benefits can thus be summed up as follows:

the better solvent properties of the water in supercritical conditionsimprove the wettability characteristics of the solid materials ofasbestos or materials containing asbestos accelerating the penetrationprocesses of the “reagent” fluid. All this leads to a marked increase inthe kinetics of the neutralising process;

the treatment procedure allows work to be carried out in a limitedenvironment, reducing the risk of emission into the environment to aminimum. It is a zero emission process;

in comparison to other processes based on etching, the absence ofchemical reagents or other substances with high environmental impact(for example, strong acids and hydrofluoric acid) make the processparticularly advantageous both from the point of view of the safety ofexecution and from the point of view of the impact on the environment.It is a process that can be ecologically supported;

the process is characterised by low energy consumption in that effectiveenergy recuperation can be made;

the not particularly aggressive working environment from the chemicalpoint of view places minor restrictions on the choice of the plantbuilding materials thus contributing to the reduction of the fixed plantcosts;

the complete neutralising of asbestos and ACW waste;

greater flexibility in executing the process;

significant reduction in the volume of the waste whether it be asbestosor ACW material;

the possibility of making both asbestos and ACW materials inertdefinitively with a single process, obtaining an inert product with acertain market value: forsterite.

The innovative process object of the present invention provides foroperating at relatively low temperatures (400-750° C.) against the 900°C. and over of the traditional thermal treatments, at relatively highpressures (22-28 MPa).

The hydrolysis time, and thus the duration of the transformationprocess, to obtain a final product without any toxic-noxious residual isless than 24 hours according to the operative conditions chosen. This isa relatively brief time if compared with the contact time proposed byother alternative neutralising processes (both thermal and chemical)that provide for more than 24 hours of work.

Another strong point of the invention is that fact that, differentlyfrom other chemical treatments, the hydrothermal process of hydrolysisin supercritical water does not require the use of any chemical reagentor substance with a high impact on the environment. In fact, the wateror the hydrogen peroxide represents the only “reagent” needed for theneutralising procedure.

The process presents the big advantage of containing and preventing thedispersion of the materials treated (asbestos or ACW) in theenvironment. Being very compact, the process can be carried out as afixed plant or as a mobile plant; in the latter case there would be theadvantage of not having to transport the dangerous waste but theneutralising could be carried out directly on the site.

The energy advantages that are obtained by using supercritical water areenormous, as efficient thermal recuperation can be provided for in theprocess that allows a significant lowering of the operative costs.

In comparison to the noise level of a normal thermal process with aconventional furnace, thanks to the absence of burners and comburent airfans, the sound emissions are almost totally absent.

Another strong point of the neutralising process proposed is thepossibility of resolving definitively, safely and not temporarily thedisposal of asbestos and of ACW waste, differently from that obtainedinstead by dumping. In addition, the possibility of avoiding disposal bydumping would avoid the construction of new special dumps that are moreand more difficult to create with a consequent saving on costs for thecommunity.

The validity and efficiency of the new process has been shown, for thefirst time, also for ACW waste with high concentrations of asbestos(fire-proof covering of asbestos-chrysotile). As the process is verysimple it can be applied industrially with massive significantquantities of ACW.

These and other characteristics of the present invention will be madeeven more evident from the following detailed description of anembodiment thereof illustrated as non-limiting example in the encloseddrawings, in which:

FIG. 1 shows the diagram of a plant according to the present invention;

FIG. 2 shows an image produced with the SEM technique (Scanning ElectronMicroscopy) of a sample of asbestos containing fibrous chrysotile;

FIG. 3 shows a spectrum produced with the EDS technique (EnergyDispersive Spectrum) of the sample of asbestos of FIG. 2;

FIG. 4 shows an enlarged image (50×) with SEM technique of the sample ofasbestos before the hydrothermal treatment according to the presentinvention;

FIG. 5 shows an enlarged image (50×) with SEM technique of the sample ofasbestos of FIG. 4 after the hydrothermal treatment according to thepresent invention;

FIG. 6 shows an enlarged image (2000×) with SEM technique of the sampleof asbestos before the hydrothermal treatment according to the presentinvention;

FIG. 7 shows an enlarged image (2000×) with SEM technique of the sampleof asbestos of FIG. 4 after the hydrothermal treatment according to thepresent invention;

FIG. 8 shows an enlarged image (3000×) with SEM technique of the sampleof asbestos before the hydrothermal treatment according to the presentinvention;

FIG. 9 shows an enlarged image (3000×) with SEM technique of the sampleof asbestos of FIG. 4 after the hydrothermal treatment according to thepresent invention;

FIG. 10 shows a spectrum produced with EDS technique of the sample ofasbestos of FIG. 4 after the hydrothermal treatment according to thepresent invention;

FIG. 11 shows a spectrum produced with XRD technique (X-Ray Diffraction)of the sample of asbestos before the hydrothermal treatment according tothe present invention;

FIG. 12 shows a spectrum produced with XRD technique of the sample ofasbestos after the hydrothermal treatment according to the presentinvention.

A plant for a hydrothermal treatment of materials containing asbestos(FIG. 1) comprises conduits 1 and a tank 2 containing distilled water,which is withdrawn from here by means of a pump 3 and conveyed to afluidised bed electric furnace 4.

Said electric furnace 4 contains a part of the conduits 1, a preheatingserpentine coil 5 and control sensors 6-7 upstream and downstream of anextractable reactor (water storage tank) 8, that is made up of twocylindrical bodies 10 held together, along the external edge, by meansof bolts 111 and in the centre a cylindrical input opening 12. In thelower part of the body 10 there is a cylindrical output opening 13.

Downstream from the electric furnace 4 there is a cooling exchanger 14with a serpentine coil 20, a filter 15 and an adjustment valve 16, thatregulates the pressure of the water which, at the end of thehydrothermal process according to the present invention, finishes in acollection tank 17.

All the abovementioned components of the plant, except for the tanks 2and 17 and the valve 16, are preferably inserted in a confined closedenvironment 50, so as to reduce the risk of emission into the air asmuch as possible.

In the operation, which can come about both in the continuous modalityand in the discontinuous modality or even “semi-batch” (that is withwater in the continuous modality and solid material in the discontinuousmodality), the pump 3 withdraws distilled water from the tank 2 andconveys it into the electric furnace 4 through conduits 1. By means ofthe preheating serpentine coil 5 the water is conveyed in supercriticalconditions (T=680° C. about, P=27 MPa) detected by the sensor 6. Thepressure is regulated by the valve 16.

The water in supercritical conditions enters through the opening 12 intothe reactor 8 loaded with asbestos or ACW material.

Said supercritical water, in contact with the asbestos or the ACWmaterial for 3 hours, penetrates the braided bands of the asbestos(FIGS. 2, 4, 6 and 8), determines a hydrolysis process that separatesthe fibres from each other, degrades the silicon chrysotile (SiO₂), thatbecomes a solution, and modifies the structure with fibrous morphologyinto crystals of forsterite (FIGS. 5, 7 and 9).

The reaction that comes about can be represented by the followingstoichiometry:2*[Mg₃Si₂O₅(OH)₄]===>3*[Mg₂SiO₄]+[SiO₂]_(aq)+4*[H₂O]

When the reaction has finished the supercritical water leaves thereactor 8 and the furnace 4 to go and cool down to an ambienttemperature in the exchanger 14.

Thanks to the filter 15 any solid materials present (asbestos or ACW)carried along are held, so that the water finishes in the collectiontank 17 simply enriched with SiO₂ and other salts, for example MgO.

Herein below, as proof of the effectiveness of the invention, theresults relating to a test carried out in the following operativeconditions are given:

the flow of the solution entering was set at 7.5° C./min;

0.15 gr of pure chrysotile were used (sample of a fireproof covering);

the temperature of the furnace 4 was regulated so as to guarantee anoperating temperature of the water storage tank 8 of 680° C.;

the micrometric valve 16 was regulated to guarantee an operatingpressure of 27 MPa;

the test was carried out using a hydrolysis time of 3 hours.

The morphology of the fibrous bodies was characterised using the SEMtechnique, which, because of its high resolutive power, allows thedetection of even the smallest dimensioned fibres. The chemical analysis(quality/quantity) of the elements was also carried out using an X-raymicroprobe. The analysis of the solid residue was carried out by meansof RX diffractometry. The watery solution collected in output from thereactor was analysed with the Icp plasma technique.

The solid extracted from the reactor after the hydrothermal hydrolysistreatment in supercritical water was weighed finding a loss in weightequalling 25%. The analyses made on the filtering cartridge revealedtotal absence of solid material, which proves that the loss in weightfound was due to the hydrolysis process. The solid collected by thewater storage tank 8 was characterised with the techniques SEM, EDS andXRD.

FIG. 2 shows a SEM image, while FIG. 3 shows the relative spectrum EDSof the sample of original chrysotile.

In the following Figures the SEM photos at various enlargements aregiven, 50× (FIGS. 4-5), 2000× (FIGS. 6-7) and 3000× (FIGS. 8-9), of thesample of asbestos before (FIGS. 4, 6 and 8) and after the hydrothermalhydrolysis treatment in supercritical water (FIGS. 5, 7 and 9).

The SEM photos taken at the highest resolutions (2000-3000×) of thesample treated (FIGS. 7 and 9) show that, by effect of the treatment,the fibrous-needle-like morphology with length equal to or greater than5 μm and diameter less than 3 μm, responsible for the toxicity of theoriginal material, has been completely transformed. In fact, no fibrewhatsoever is visible in the solid and thus it can be concluded that thehydrothermal hydrolysis process in supercritical water has allowed theoriginal fibrous asbestos to be transformed into a non-fibrous andnon-toxic phase. The EDS analysis, shown in FIG. 10 was carried out onthe solid. The ratio between the height of the peaks of the Mg and thoseof the Si are significantly different before (FIG. 3) and after thetreatment (FIG. 10). Before the treatment the ratio of the heights Mg/Siis equal to 1.3, this means that the height of the peak of the Mg in thechrysotile is just over that of the peak of the Si; while in thespectrum of the sample treated the same ratio equals 2.1; this meansthat the spectrum of FIG. 10 is not representative of the chrysotile.This conclusion is also supported by the fact that the spectrum of FIG.10 indicates the complete absence of iron, another characteristicchemical element, even though present in a small concentration inchrysotile.

After the treatment RX diffractometric analyses were conducted tohighlight the presence of crystalline substances and their nature. FIGS.11 and 12 show the two spectrums of the sample before (FIG. 11) andafter the treatment (FIG. 12). Both spectrums reveal that inside thesolid material there is a monocrystalline phase. While the spectrum ofFIG. 11 is characteristic of the chrysotile, that of FIG. 12 ischaracteristic of the forsterite.

In the sample hydrolysed with supercritical water and analysed with theXRD technique the presence of chrysotile was not detected, which, ifpresent, was in concentrations that are lower than the limit ofdetection of the instrument. The total absence of fibres, thatdemonstrates the effectiveness of the innovative treatment proposed, wasdemonstrated by the SEM, which, also at greater resolutions did nothighlight the presence of fibrous solid.

To make the process more effective and attractive, the asbestos and/orthe material containing asbestos can be given a wet pretreatment. Inparticular said asbestos and/or material containing asbestos can bebroken up and ground in the presence of water, with or without additive,until it reaches a preset consistence (for example from 20% to 30% solidpart), then it is loaded into the reactor.

In this manner dispersion of asbestos fibre in the working environmentand the inconvenience of working with anhydrous material or however withhumidity lower than 10-20% are avoided, which are typical conditions ofthermal treatments of ACW materials.

To treat materials containing asbestos with organic matrix an oxidisingenvironment of supercritical water can be used for operation.

Finally it can be noted that the process and the plant described abovein relation to asbestos and to materials containing asbestos must beconsidered conceptually valid and also applicable to any other materialwith similar characteristics, in particular for neutralising anypotentially dangerous material with fibrous morphology.

1. Process for the hydrothermal treatment of asbestos and/or materialscontaining asbestos in supercritical water (Supercritical Water, SCW)characterised in that in succession they provide for the followingsteps: withdrawal (3) of water from a tank (2); transformation (5) ofthe water in supercritical water; reaction of the supercritical waterwith asbestos and/or with the material containing asbestos in a suitableenvironment (8) by means of a hydrolysis process; cooling (14) of thewaste water; filtering (15) of the waste water; collection of the wastewater in a tank (17).
 2. Process according to claim 1, characterised inthat said water is distilled.
 3. Process according to claim 1,characterised in that said water is oxygenate.
 4. Process according toclaim 1, characterised in that it is produced in a confined environment.5. Process according to claim 1, characterised in that it is carried outin continuous modality.
 6. Process according to claim 1, characterisedin that it is carried out in discontinuous modality.
 7. Processaccording to claim 1, characterised in that it is carried out insemi-batch modality, that is with water in continuous modality and solidmaterial in discontinuous modality.
 8. Process according to claim 1,characterised in that it is carried out in the following conditions oftemperature T, pressure P and hydrolysis time: 400° C.<T<750° C.; 22.11MPa<P<28 MPa.; hydrolysis time <24 hours.
 9. Process according to claim1, characterised in that, before the reaction with the supercriticalwater, the asbestos and/or the material containing asbestos is submittedto a wet pretreatment.
 10. Process according to claim 9, characterisedin that said wet pretreatment provides for the breaking up and grindingof the asbestos and/or of the material containing asbestos in thepresence of water.
 11. Process according to claim 9, characterised inthat said pretreatment comes about with additives.
 12. Plant for thetreatment of asbestos and/or materials containing asbestos characterisedin that it comprises a water tank (2), a withdrawal pump (3) associatedto said tank (2), a furnace (4) containing a serpentine coil (5) fed bysaid withdrawal pump (3) for the transformation of the water insupercritical water and a reactor (8) for the reaction of thesupercritical water with asbestos and/or with the material containingasbestos, heat exchange means (14) for cooling the waste water of saidreactor (8), and means for filtering (15) the water positioned at theoutput of said exchange means (14) collection means (17) of the cooledand filtered waste water.
 13. Plant according to claim 12, characterisedin that said water is distilled.
 14. Plant according to claim 12,characterised in that said water is oxygenated.
 15. Plant according toclaim 12, characterised in that it is in a confined environment. 16.Plant according to claim 12, characterised in that the reactor (8) ismade to function with the following values of temperature T, of pressureP and hydrolysis time: 400° C.<T<750° C.; 22.11 MPa<P<28 MPa.;hydrolysis time <24 hours.
 17. Plant according to claim 12,characterised in that the furnace (4) is electric.
 18. Plant accordingto claim 12, characterised in that said furnace (4) is a fluidised bed.19. Plant according to claim 12, characterised in that said reactor (8)is a water storage tank.
 20. Plant according to claim 12, characterisedin that said reactor (8) can be extracted from the furnace (4). 21.Plant according to claim 12, characterised in that said exchangers (14)have a serpentine coil.
 22. Plant according to claim 12, characterisedin that it comprises an adjustment valve (16) inserted downstream fromsaid heat exchange means (14).